Because exhaust gases discharged from internal engines such as diesel engines, gasoline engines, etc. contain harmful substances such as nitrogen oxide (NOx) and particulate matter (PM), exhaust pipes of the internal engines are provided with units for removing particulate matter, and units for removing nitrogen oxide. The nitrogen-oxide-removing units include an urea-SCR catalyst, in which urea injected into an exhaust pipe is turned to ammonia, which is reacted with nitrogen oxide in the exhaust gas to remove oxygen therefrom, thereby reducing nitrogen oxide to nitrogen, and thus removing nitrogen oxide from the exhaust gas. Attention is also paid to an HC-SCR catalyst technology using a diesel fuel (HC) as a reducing agent, which can be used without needing sufficient urea-supplying facilities.
An example of ceramic honeycomb structures used as a carrier for the SCR catalyst is shown in FIGS. 1 and 2. A ceramic honeycomb structure 10 comprises porous cell walls 2 defining large numbers of exhaust-gas-flowing paths 3, and an outer peripheral wall 1, with a catalytic material (not shown) carried by the porous cell walls 2.
To remove nitrogen oxide from the exhaust gas efficiently, as large an amount of a catalytic material as possible should be carried per a unit volume, such that the catalytic material carried by an SCR catalyst carrier comes into sufficient contact with the exhaust gas. Conventionally used to this end is an SCR catalyst comprising as a carrier a ceramic honeycomb structure having thin walls with a high cell density (for example, cell wall thickness: 0.05 mm, and cell wall pitch: 0.85 mm). However, such a honeycomb structure having thin walls with a high cell density has a small opening area in each exhaust-gas-flowing cell, suffering large pressure loss at its inlet.
To solve such a problem as increased pressure loss, JP 2005-052750 A discloses a ceramic honeycomb structure comprising cell walls having a thickness of 0.1-0.35 mm, a pitch of 1.0-2.0 mm, an average pore diameter of 15 μm or more, and porosity of 50-80%. JP 2005-052750 A describes that by optimizing the porosity and average pore diameter of cell walls of a honeycomb structure as a catalyst carrier without making the cell walls thinner with a higher cell density, the amount of a catalytic material carried per a unit volume can be increased to improve the cleaning efficiency of a NOx-removing ceramic honeycomb catalyst such as an SCR catalyst, and to reduce its size.
JP 2009-542570 A discloses a cordierite ceramic product having porosity of 64% or more and less than 80%, a median pore diameter d50 of 10-45 μm, a thermal expansion coefficient CTE of 3.0×10−7/° C. or more, and (i) CTE of less than 6.0×10−7/° C. at a median pore diameter d50 of 10 μm or more and less than 18 μm, (ii) CTE of less than 9.0×10−7/° C. at a median pore diameter d50 of 18 μm or more and less than 22 μm, (iii) CTE of less than 10.0×10−7/° C. at a median pore diameter d50 of 2-25 μm, (iv) CTE of less than 13.0×10−7/° C. at a median pore diameter d50 of more than 25 μm and less than 29 μm, and (v) CTE of less than 17.0×10−7/° C. at a median pore diameter d50 of 29-45 μm. JP 2009-542570 A describes that this ceramic product has drastically improved breakage strength coefficient and heat shock resistance despite high porosity, and that even with effective amounts of a catalyst and/or a NOx-absorbing material coated, the finely porous ceramic structure secures low pressure loss during cleaning and soot accumulation, thereby making the cordierite ceramic product suitable for catalyst-carrying, wall-flow filters for diesel particles. JP 2009-542570 A further describes that a narrow pore diameter distribution enables a more uniform distribution of a catalyst on pore wall surfaces, resulting in low pressure loss during cleaning and soot accumulation, providing increased chances of contacting the catalyst with soot and the exhaust gas, and thus higher use efficiency of the catalyst.
JP 2011-516371 A discloses a porous polycrystalline ceramic body having an anisotropic microstructure composed of oriented polycrystalline reticular formations, with an anisotropic factor (Af-pore long) meeting 1.2<Af-pore long<5, which can provide a ceramic article having a narrow pore diameter distribution, porosity of more than 50%, and a median pore diameter in a range of 12-25 μm. JP 2011-516371 A describes that this ceramic article exhibiting high strength, a low thermal expansion coefficient (CTE) and high porosity can be used for substrates of diesel or gasoline particulate filters, and functional filters such as catalyst filters having partial or complete NOx-adding functions, in automobiles.
WO 2011/102487 discloses a ceramic honeycomb structure comprising cell walls having (a) porosity of 55-80%, (b) a median pore diameter d 50 (measured by mercury porosimetry) of 5-27 μm, (c) pores open on the surface having an opening area ratio of 20% or more, (d) an area-based median opening diameter d50 (corresponding to an average equivalent circle diameter of pores open on the surface) of 10-45 μm, (e) the density of pores (having equivalent circle diameters of 10 μm or more and less than 40 μm) open on the surface being 350/mm2 or more, (f) the maximum inclination of a curve of a cumulative pore volume to a pore diameter being 1.6 or more in a pore diameter distribution measured by mercury porosimetry, and (g) a ratio D50/d50 of the median pore diameter D50 to the median opening diameter d50 being 0.65 or less. WO 2011/102487 describes that a ceramic honeycomb filter comprising this ceramic honeycomb structure effectively captures nano-particles largely affecting the number of particles discharged even before PM is accumulated at an initial stage of use, resulting in an improved number-based capturing ratio of PM, with less deterioration of pressure loss characteristics when PM is accumulated.
WO 2011/027837 discloses a ceramic honeycomb structure comprising cell walls having porosity of 40-60%; the opening area ratio of pores open on the cell wall surfaces (the total opening area of pores per a unit cell wall surface area) being 15% or more; the area-based median opening diameter of the open pores being 10 μm or more and less than 40 μm, when the opening diameter of each pore open on the cell wall surfaces is expressed by an equivalent circle diameter (diameter of a circle having the same area as the opening area of a pore); the density of pores having equivalent circle diameters of 10 μm or more and less than 40 μm being 350/mm2 or more; and the average circularity of pores having equivalent circle diameters of 10 μm or more and less than 40 μm being 1-2. WO 2011/027837 describes that because the ceramic honeycomb structure exhibits an improved PM-capturing ratio while keeping low pressure loss, at an early capturing stage after regeneration, it can efficiently capture nano-sized PM, which gathers attention under increasingly stricter exhaust gas regulations.
However, an SCR catalyst comprising as a carrier the ceramic honeycomb structure described in JP 2005-052750 A, the cordierite ceramic product described in JP 2009-542570 A, the porous ceramic body described in JP 2011-516371 A, and the ceramic honeycomb structure used in the ceramic honeycomb filters described in WO 2011/102487 and WO 2011/027837 fail to exhibit satisfactorily high cleaning efficiency under the recent demand of higher cleaning performance and higher efficiency, despite somewhat improved pressure loss characteristics and nitrogen-oxide-removing efficiency. When the amount of a catalytic material carried on cell walls is increased to obtain high cleaning efficiency, exhaust-gas-flowing paths have smaller opening areas, resulting in larger exhaust-gas-flowing resistance, and thus larger pressure loss. Also, an SCR catalyst comprising as a carrier the ceramic honeycomb structure used in the ceramic honeycomb filter described in WO 2011/102487 may have insufficient strength.